This application relates generally to microelectromechanical systems, and more particularly to MEMS devices and methods for use in micromirror arrays.
In recent years, increasing emphasis has been made on the development of techniques for producing microscopic systems that may be tailored to have specifically desired electrical and/or mechanical properties. Such systems are generically described as microelectromechanical systems (MEMS) and are desirable because they may be constructed with considerable versatility despite their very small size. In a variety of applications, MEMS component structures may be fabricated to move in such a fashion that there is a risk of stiction between that component structure and some other aspect of the system. One such example of a MEMS component structure is a micromirror, which is generally configured to reflect light from at least two positions. Such micromirrors find numerous applications, including as parts of optical switches, display devices, and signal modulators, among others.
In many applications, such as may be used in fiber-optics applications, such MEMS-based devices may include hundreds or even thousands of micromirrors arranged as an array. Within such an array, each of the micromirrors should be accurately aligned with both a target and a source. Such alignment is generally complex and typically involves fixing the location of the MEMS device relative to a number of sources and targets. If any of the micromirrors is not positioned correctly in the alignment process and/or the MEMS device is moved from the aligned position, the MEMS device will not function properly.
In part to reduce the complexity of alignment, some MEMS devices provide for individual movement of each of the micromirrors. An example is provided in FIGS. 1A-1C illustrating a particular MEMS micromirror structure that may take three positions. Each micromirror includes a reflective surface 116 mounted on a micromirror structural film 112 that is connected by a structural linkage 108 to an underlying substrate 104. Movement of an individual micromirror is controlled by energizing actuators 124a and/or 124b disposed underneath the micromirror on opposite sides of the structural linkage 108. Hard stops 120a and 120b are provided to stop the action of the micromirror structural film 112. Energizing the actuator 124a on the left side of the structural linkage 108 causes the micromirror to tilt on the structural linkage 108 towards that side until one edge of the micromirror structural film 112 contacts the left hard stop 120a, as shown in FIG. 1A. Alternatively, the actuator 124b on the right side of the structural linkage 108 may be energized to cause the micromirror to tilt in the opposite direction, as shown in FIG. 1B. When both actuators are de-energized, as shown in FIG. 1C, the micromirror returns to a static position horizontal to the structural linkage 108. In this way, the micromirror may be moved to any of three positions. This ability to move the micromirror provides a degree of flexibility useful in aligning the MEMS device, although the alignment complexity remains significant. Sometimes hard stops 120a and 120b are not provided so that the micromirror structural film 112 is in direct contact with the substrate 104.
Even in such configurations, however, the angle of the micromirror in a tilted position is fixed. Processes for fabrication of MEMS structures are such that this tilt angle may not be ideal for a given application. Even relatively small deviations from the preferred tilt angle, when present for numerous micromirrors in an array, may result in performance for a system that is less than ideal. Moreover, in some such applications, the micromirror may remain in a given tilted position for ten years or more. Thus, for example, one side of an individual micromirror structural film may remain in contact with the hard stop or substrate for extended periods. Maintaining such contact increases the incidence of dormancy-related stiction. Such stiction results in the micromirror remaining in a tilted position even after the actuators are de-energized. Some theorize that stiction is a result of molecule and/or charge build up at the junction between the micromirror structural film and the hard stop or substrate. For example, it has been demonstrated that an accumulation of H2O molecules at the junction produces capillary forces that increase the incidence of stiction.
Some solutions exist for overcoming stiction, such as by packaging the MEMS device in a hermetic environment to reduce molecular accumulation at the junction or by periodically vibrating the device, such as described in Ville Kaajakari, xe2x80x9cUltrasonic Actuation for MEMS Dormancy-Related Stiction Reductionxe2x80x9d, Proceedings of SPIE Vol. 4180 (2000), which is herein incorporated by reference for all purposes. Such techniques, however, do not address the fact that the tilt angles for the micromirrors are fixed. Moreover, stiction remains a concern is such solutions; it would be preferable to use a method for dynamically selecting individual angles for each mirror in an array without the micromirror structural film ever contacting a hard stop or substrate.
Embodiments of the invention are thus directed to a method and system that permits individual tilt angles to be selected for optical micromirrors within an array. Each optical micromirror in the array includes a structural film connected with a substrate by a structural linkage. At least one electrode is configured to tilt the structural film upon application of a voltage.
In one embodiment, a method is provided for operating an array of such optical micromirrors. Electrodes associated with each of a plurality of optical micromirrors within the array are sequentially actuated by applying a voltage to each such electrode for a fixed time. The voltage applied to each of the electrodes is selected so that the optical micromirror with which that electrode is associated is positioned in a certain tilted position. The step of sequentially actuating electrodes is repeated to maintain the tilted positions of the plurality of optical micromirrors. In one embodiment, the voltage selected for each of the electrodes is less than a snap-in voltage for its associated optical micromirror. One of the plurality of optical micromirrors may comprise a torsion-beam micromirror. Alternatively, one of the plurality of optical micromirrors may comprise a cantilever micromirror. The voltage applied to each of the electrodes may differ, as may the fixed time for which the voltage is applied. These voltages and/or fixed times may be retrieved from a memory and may be changed as necessary or desirable, such as in response to external conditions. One of the micromirrors may be switched to a new tilted position by changing the applied voltage or may be switched by resonantly driving the associated electrode.
In another embodiment, at least one of the plurality of optical micromirrors uses a comb-drive linear-actuation configuration. In such a configuration, a first actuator linkage is connected with the structural film on a first side of the structural linkage and connected with a first linear actuator on a second side of the structural linkage. Similarly, a second actuator linkage is connected with the structural film on the second side of the structural linkage and connected with a second linear actuator on the first side of the structural linkage. A first electrode is configured, such as with a comb drive, to move the first linear actuator upon actuation and a second electrode is similarly configured to move the second linear actuator upon actuation.
Such embodiments for a method of the invention may be realized with a system that includes a multichannel driver and a controller. In one embodiment, the multichannel driver has a plurality of lines in communication with the electrodes associated with the plurality of optical micromirrors. It is configured to apply a voltage through such lines to the electrodes. The controller is configured to control the multichannel driver to maintain the plurality of optical micromirrors in a set of certain tilted positions. This is done by sequentially actuating the electrodes by applying the voltage for each electrode for the fixed time. Where such voltages and/or fixed times are retrieved from a memory, the system may also comprise a memory in communication with the controller. In one embodiment, the memory is a digital memory and the system further comprises a digital-analog converter.
In some embodiments, the array of optical micromirrors is part of a wavelength router for directing a plurality of spectral bands to a plurality of output ports. In such embodiments, the tilted positions of the plurality of optical micromirrors may approximately equalize the intensity of the plurality of spectral bands at the plurality of output ports.